Convert AU to ALAC — Free Online Tool
Convert Sun AU audio files to Apple Lossless (ALAC) in your browser — transforming Unix-era PCM audio into a modern, losslessly compressed M4A file compatible with iTunes, Apple Music, and all Apple devices. The original PCM audio data is re-encoded using the ALAC codec with zero quality loss.
to
FFmpeg Command
Copy this command to run the same conversion locally with FFmpeg on your desktop. Download FFmpeg
Drop your AU file here
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Settings
Note: Browser-based encoding uses approximate quality targets. For precise CRF compression, copy the FFmpeg command above and run it on your desktop.
Estimated output:
Conversion Complete!
DownloadHow It Works
AU files store raw PCM audio (typically 16-bit big-endian signed PCM, as used on Sun/NeXT workstations) with a minimal header. During this conversion, FFmpeg reads the raw PCM stream from the AU container and re-encodes it using Apple Lossless Audio Codec (ALAC), which applies lossless compression — mathematically identical to the original samples but with a smaller file size. The output is wrapped in an MPEG-4 (.m4a) container, which adds support for metadata tags and chapter markers absent in the AU format. Because both the source and destination are lossless (PCM and ALAC respectively), no audio quality is lost at any stage — every sample value is preserved exactly.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg media processing engine. In the browser tool, this runs as FFmpeg.wasm compiled to WebAssembly — the same command works identically on a desktop FFmpeg installation. |
-i input.au
|
Specifies the input Sun AU file. FFmpeg detects the AU container from the file header (magic bytes 0x2e736e64) and identifies the PCM encoding type (s16be, mulaw, alaw, etc.) automatically from the AU encoding field. |
-c:a alac
|
Sets the audio codec to ALAC (Apple Lossless Audio Codec) for encoding. This instructs FFmpeg to losslessly compress the raw PCM audio from the AU file into ALAC, which is the required codec for the .m4a output container in this conversion. |
-c:a alac
|
A duplicate of the preceding -c:a alac flag in the resolved command — this is redundant but harmless, as FFmpeg applies the last codec specification. No functional difference results; the output is still encoded with ALAC. |
output.m4a
|
Defines the output file as an MPEG-4 audio container (.m4a). FFmpeg uses this extension to select the MP4/M4A muxer, which is the standard and Apple-supported container for ALAC audio, enabling compatibility with iTunes, Apple Music, and iOS devices. |
Common Use Cases
- Archiving legacy Unix workstation audio recordings or NeXT computer sound files into a modern lossless format playable on Apple devices without quality degradation.
- Importing classic Sun AU sound effects or samples into GarageBand or Logic Pro, which natively support ALAC/M4A but may not recognize raw AU files.
- Converting AU audio from early internet-era software distributions or game assets into a format compatible with iTunes and Apple Music libraries.
- Preparing AU-format voiceovers or speech recordings from Unix telephony systems (often stored as pcm_mulaw or pcm_alaw) into ALAC for long-term lossless archival.
- Migrating AU audio assets from older scientific or research software into a format that supports rich metadata tagging (artist, album, track number) via the M4A container.
- Producing an Apple-ecosystem-compatible lossless master from raw PCM AU files before distributing to services that accept ALAC uploads.
Frequently Asked Questions
No — this is a fully lossless conversion in both directions. AU files contain raw PCM audio (uncompressed), and ALAC is a lossless compression codec, meaning every audio sample is encoded and decoded identically to the original. The resulting ALAC file will sound bit-for-bit identical to the source AU file. The only transformation is that the data is compressed to save space, not approximated.
ALAC is a codec, not a container — it needs to be stored inside a container format. Apple standardized ALAC storage inside the MPEG-4 container, giving the file a .m4a extension. This container is what allows the file to carry metadata tags (title, artist, album art) and be recognized by iTunes, Apple Music, Finder, and iOS devices. The AU format, by contrast, has an extremely minimal header with no metadata support.
Yes. FFmpeg automatically decodes mu-law (pcm_mulaw) and A-law (pcm_alaw) AU files — both common in Unix telephony systems — into linear PCM internally before re-encoding to ALAC. The output ALAC file will contain standard linear PCM audio, which is the highest-fidelity representation of that audio. Note that mu-law and A-law are themselves lossy compression formats with 8-bit resolution, so the ALAC output faithfully preserves that quality level but cannot recover information not present in the original.
For typical speech and music content, ALAC lossless compression reduces file size by roughly 40–60% compared to uncompressed 16-bit PCM AU files. The exact ratio depends on the complexity of the audio — highly dynamic music compresses less than speech or silence-heavy recordings. AU files using 8-bit PCM may actually result in ALAC files of comparable size due to the lower bit depth having less compressible redundancy.
The displayed command processes a single file. To batch-convert all AU files in a folder on Linux or macOS, you can use a shell loop: `for f in *.au; do ffmpeg -i "$f" -c:a alac "${f%.au}.m4a"; done`. On Windows Command Prompt, use: `for %f in (*.au) do ffmpeg -i "%f" -c:a alac "%~nf.m4a"`. This is especially useful for converting large collections of legacy sound assets, since the browser tool is optimized for individual files up to 1GB.
ALAC support outside the Apple ecosystem is more limited than formats like FLAC. Android does not natively support ALAC playback in most versions, though apps like VLC and PowerAmp handle it. On Windows and Linux, VLC and foobar2000 support ALAC. If broad cross-platform compatibility is your goal, consider converting the AU file to FLAC instead — FLAC is equally lossless but enjoys near-universal support. ALAC is the ideal choice specifically when the files will live in iTunes, Apple Music, or on iOS/macOS devices.
Technical Notes
AU files carry a simple 6-field header (magic number, data offset, data size, encoding type, sample rate, channel count) with no provision for metadata tags, album art, or chapter markers. ALAC stored in an M4A container supports all of these via MPEG-4 atoms, so the conversion opens up metadata capabilities the source format structurally cannot provide. The default AU codec (pcm_s16be — 16-bit signed big-endian PCM) maps cleanly to ALAC's internal 16-bit lossless representation. If the AU source uses 8-bit signed (pcm_s8) or unsigned 8-bit (pcm_u8) PCM, FFmpeg will correctly decode and re-encode those at their native bit depth. One known limitation: AU files occasionally lack accurate sample-count metadata in their headers (the data size field may be set to 0xFFFFFFFF for streaming), which FFmpeg handles by reading to end-of-file — this is not an issue for locally stored files but is worth noting if processing AU files originally streamed from network sources. The FFmpeg command contains a redundant `-c:a alac` flag (appearing twice), which is harmless — FFmpeg simply applies the last valid instance.